Aah, yes! That describes it much better. The guilt of being the only one left alive would create a sense of duty a lot more than it would a dream.

I could see Sasuke throw aside his duty as well, but only if it was a drastic situation. Like, it really would have to be Sasuke has the option of killing Itachi but only if he doesn't interfere to keep the rest of team seven from dying. Anything less than that, and I'd be hard pressed to believe it.

I mean, by this point, he's given up so much on his quest to gain enough power to kill Itachi, I think if he didn't succeed it would be like he wasted everything in the end. So it would be a sense of duty mixed with a need to justify his actions, maybe?